Conventionally, a rush current prevention technique is implemented for preventing a load (e.g., an inverter) that receives power supply (i.e., supply of a direct current) via a relay, by storing electric charge in advance in a smoothing capacitor that is connected in parallel with the load and by turning ON the relay afterward.
For example, a power device disclosed in a patent document 1 i.e., Japanese Patent No. 5127387, includes a pre-charge circuit that is connected in parallel with a relay. The pre-charge circuit includes a switch such as MOSFET or the like. The power device in such configuration stores electric charge in advance in a smoothing capacitor before turning ON of the relay.
The configuration that includes a pre-charge circuit must increase an electric current flowing in the pre-charge circuit for pre-charging of a large capacity capacitor in a short time. That is, such a configuration must have a large capacity semiconductor switch that allows a large electric current, which may lead to an increase of the production cost, for example.
Further, as a different configuration, a current sensing resistor disposed at a position between a direct current power source and a relay for a constant current control of the relay may also be used for the pre-charging. However, disposing a resistor in a main current path leads to an increase of the circuit loss.